The invention relates to a voltage converter for producing a controlled output voltage from an input voltage.
Voltage converters are well known in the prior art, and are described in their various constructions as step-down transformers, step-up transformers, isolating transformers, etc., in Tietze, Schenk: "Halbleiterschaltungstechnik," 9th ed., 1990, pp. 561 to 576. In voltage converters of this sort, a switching element is connected with an inductive element, e.g. a coil or the primary winding of a transformer, in order to apply an input voltage, which can be a rectified alternating voltage, pulse-by-pulse to the inductive element. Dependent on a control circuitry, the switching element is clock-controlled in pulse-width-modulated fashion in such a way that an output voltage with a desired characteristic is present. The controlled output voltage is normally supposed to be a voltage that is as constant as possible and is present in load-independent fashion.
The control circuit required for controlling the switching element is normally realized as an integrated circuit with external wiring elements for signal production and preparation. The integrated control module TDA 4605 from Siemens is suited for use as a control IC for isolating-transformer switch-mode power supplies. The signal processing in the TDA 4605 is carried out with analog circuit technology. This circuit technology has the advantage that a relatively small number of transistors is required, but has the disadvantage of difficult dimensioning and high sensitivity to disturbances in relation to operational fluctuations and the influence of parasitic elements.
In the reference Patent Abstracts of Japan, 1991, vol. 15, no. 236, (E-1078), JP 3-70464, a switching controller with a digitally operating controlling is specified. The control circuit contains a counter driven by an oscillator a counter state of which is compared with a comparison value. A switching element provided in the switching controller is controlled dependent on the comparison result. The comparison value is provided by a bidirectional counter whose bidirectional counter input is driven by a comparator, by means of which the output voltage to be controlled is compared with a reference value.
In Patent Abstracts of Japan, 1992, vol. 16, no. 316 (E-1231), JP 4-87556, a free-wheeling switch-mode power supply with a digitally operating controlling is specified, in which an operational counter, clock-controlled by an oscillator, is reset by a feedback signal derived from a transformer winding. By this means, the turn-off time of the switching transistor is matched to the resonance frequency of the transformer.
In Patent Abstracts of Japan, 1990, vol. 14, no. 95 (E-892), JP 1-298958, a switch-mode power supply is described whose control circuit contains a CPU.
The previously known digitally operating power packs are operated with unaltered functioning for low load at the output side (standby), and provide no particular circuit means that take this operational state into account. A low power loss of the power pack is desirable in standby operation in particular.